XMLHttpRequestEventTarget.onloadstart

The XMLHttpRequestEventTarget.onloadstart is the function called when an XMLHttpRequest transaction starts transferring data.

XMLHttpRequest.onloadstart = callback;

• callback is the function to be called when the transaction begins to transfer data.

var xmlhttp = new XMLHttpRequest(),
  method = 'GET',
  url = 'https://developer.mozilla.org/';

xmlhttp.open(method, url, true);
xmlhttp.onloadstart = function () {
  console.log("Download underway");
};
xmlhttp.send();